Android 自定义view 和 onMeasure方法介绍

转自：http://blog.youkuaiyun.com/cyq1028/article/details/7390631

http://blog.youkuaiyun.com/sunny2come/article/details/7287298

哈哈，兄弟我终于自己写了一个view，不是网上那种简单的哦，还是有一定技术含量的，

我是通过学习ApiDemo（android自带的sample）里面LabelView实现的，

先谈谈学习过程，觉得一开始不应当盲目的动手做，应对想把原理搞明白，哪怕一个很小的View，也应当将各个细节弄清楚，

等这些搞定了，接下来的工作就是水道渠成了！

自定义一个View那必须继承View，

首先说说我的View是啥，恩，很简单，就是一个椭圆，其中可以设置4个参数，分别是top、left、right、bottom，应该很清楚吧，因为canvas.drawOval时用到了4个值，

首先得定义4个属性值，是我的View专有的，

建立attr.xml放到values下面

[java] view plain copy print ?

1. <?xml version="1.0" encoding="UTF-8"?>
2. <resources>
3. <declare-styleable name="RocketView">
4. <attr name="ovalLeft" format="dimension" />
5. <attr name="ovalTop" format="dimension" />
6. <attr name="ovalRight" format="dimension" />
7. <attr name="ovalBottom" format="dimension" />
8. </declare-styleable>
9. </resources>

<?xml version="1.0" encoding="UTF-8"?>
<resources>
     <declare-styleable name="RocketView">
        <attr name="ovalLeft" format="dimension" />
        <attr name="ovalTop" format="dimension" />
        <attr name="ovalRight" format="dimension" />
        <attr name="ovalBottom" format="dimension" />
    </declare-styleable>
</resources>

然后写个RocketView

[java] view plain copy print ?

1. package com.myos;
3. import android.content.Context;
4. import android.content.res.TypedArray;
5. import android.graphics.Canvas;
6. import android.graphics.Color;
7. import android.graphics.Paint;
8. import android.graphics.RectF;
9. import android.util.AttributeSet;
10. import android.util.Log;
11. import android.view.View;
12. import android.view.View.MeasureSpec;
14. public class RocketView extends View{
15. private Paint mOvalPaint;
16. private int mStrokeWidth = 2;
17. private int padding = 3;
19. //椭圆参数
20. private int mOval_l;
21. private int mOval_t;
22. private int mOval_r;
23. private int mOval_b;
27. //构造
28. public RocketView(Context context, AttributeSet attrs) {
29. super(context, attrs);
30. initRocketView();
32. TypedArray a = context.obtainStyledAttributes(attrs,
33. R.styleable.RocketView);
35. mOval_l = a.getDimensionPixelOffset(R.styleable.RocketView_ovalLeft, padding);
36. mOval_t = a.getDimensionPixelOffset(R.styleable.RocketView_ovalTop, padding);
37. mOval_r = a.getDimensionPixelOffset(R.styleable.RocketView_ovalRight, 100);
38. mOval_b = a.getDimensionPixelOffset(R.styleable.RocketView_ovalBottom, 100);
40. a.recycle();
41. }
43. private void initRocketView() {
44. mOvalPaint = new Paint();
45. mOvalPaint.setAntiAlias(true);
46. mOvalPaint.setColor(Color.BLUE);
47. mOvalPaint.setStyle(Paint.Style.STROKE);
48. mOvalPaint.setStrokeWidth(mStrokeWidth);
49. setPadding(padding,padding,padding,padding);
50. }
52. public void setOvalRect(int l, int t, int r, int b){
53. mOval_l = l + padding;
54. mOval_t = t + padding;
55. mOval_r = r;
56. mOval_b = b;
57. requestLayout();
58. invalidate();
59. }
60. @Override
61. protected void onDraw(Canvas canvas) {
62. // TODO Auto-generated method stub
63. super.onDraw(canvas);
64. canvas.drawColor(Color.WHITE);
65. // 绘制椭圆
66. RectF re11 = new RectF(mOval_l, mOval_t, mOval_r, mOval_b);
67. canvas.drawOval(re11, mOvalPaint);
69. // // 绘制圆形
70. // canvas.drawCircle(mCircle_x, mCircle_y, mCircle_r, mPaint);
71. }
73. /**
74. * @see android.view.View#measure(int, int)
75. */
76. @Override
77. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
78. setMeasuredDimension(measureWidth(widthMeasureSpec),
79. measureHeight(heightMeasureSpec));
80. }
82. /**
83. * Determines the width of this view
84. * @param measureSpec A measureSpec packed into an int
85. * @return The width of the view, honoring constraints from measureSpec
86. */
87. private int measureWidth(int measureSpec) {
88. int result = 0;
89. int specMode = MeasureSpec.getMode(measureSpec);
90. int specSize = MeasureSpec.getSize(measureSpec);
92. if (specMode == MeasureSpec.EXACTLY) {
93. // We were told how big to be
94. result = specSize;
95. } else {
96. // Measure the text
97. result = mOval_r + getPaddingLeft()
98. + getPaddingRight();
99. if (specMode == MeasureSpec.AT_MOST) {
100. // Respect AT_MOST value if that was what is called for by measureSpec
101. result = Math.min(result, specSize);
102. }
103. }
105. return result;
106. }
108. /**
109. * Determines the height of this view
110. * @param measureSpec A measureSpec packed into an int
111. * @return The height of the view, honoring constraints from measureSpec
112. */
113. private int measureHeight(int measureSpec) {
114. int result = 0;
115. int specMode = MeasureSpec.getMode(measureSpec);
116. int specSize = MeasureSpec.getSize(measureSpec);
118. if (specMode == MeasureSpec.EXACTLY) {
119. // We were told how big to be
120. result = specSize;
121. } else {
122. // Measure the text (beware: ascent is a negative number)
123. result = mOval_b + getPaddingTop()
124. + getPaddingBottom();
125. if (specMode == MeasureSpec.AT_MOST) {
126. // Respect AT_MOST value if that was what is called for by measureSpec
127. result = Math.min(result, specSize);
128. }
129. }
131. return result;
132. }
133. }

package com.myos;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.view.View.MeasureSpec;

public class RocketView extends View{
	private Paint mOvalPaint;
	private int mStrokeWidth = 2;
	private int padding = 3;
	
	//椭圆参数
	private int mOval_l;
	private int mOval_t;
	private int mOval_r;
	private int mOval_b;
	
	
	
	//构造
	public RocketView(Context context, AttributeSet attrs) {
		super(context, attrs);
		initRocketView();
		
		TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.RocketView);

        mOval_l = a.getDimensionPixelOffset(R.styleable.RocketView_ovalLeft, padding);  
        mOval_t = a.getDimensionPixelOffset(R.styleable.RocketView_ovalTop, padding);  
        mOval_r = a.getDimensionPixelOffset(R.styleable.RocketView_ovalRight, 100);  
        mOval_b = a.getDimensionPixelOffset(R.styleable.RocketView_ovalBottom, 100);  

        a.recycle();
	}

    private void initRocketView() {
    	mOvalPaint = new Paint();
    	mOvalPaint.setAntiAlias(true);
    	mOvalPaint.setColor(Color.BLUE);
    	mOvalPaint.setStyle(Paint.Style.STROKE);
    	mOvalPaint.setStrokeWidth(mStrokeWidth);
    	setPadding(padding,padding,padding,padding);
    }
    
    public void setOvalRect(int l, int t, int r, int b){
    	mOval_l = l + padding;
    	mOval_t = t + padding;
    	mOval_r = r;
    	mOval_b = b;
    	requestLayout();
        invalidate();
    }
	@Override
	protected void onDraw(Canvas canvas) {
		// TODO Auto-generated method stub
		super.onDraw(canvas);
		canvas.drawColor(Color.WHITE);
		// 绘制椭圆
		RectF re11 = new RectF(mOval_l, mOval_t, mOval_r, mOval_b);
		canvas.drawOval(re11, mOvalPaint);	
		
//		// 绘制圆形
//		canvas.drawCircle(mCircle_x, mCircle_y, mCircle_r, mPaint);
	}
	
	 /**
	  * @see android.view.View#measure(int, int)
	  */
	 @Override
	 protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
	     setMeasuredDimension(measureWidth(widthMeasureSpec),
	             measureHeight(heightMeasureSpec));
	 }

	 /**
	  * Determines the width of this view
	  * @param measureSpec A measureSpec packed into an int
	  * @return The width of the view, honoring constraints from measureSpec
	  */
	 private int measureWidth(int measureSpec) {
	     int result = 0;
	     int specMode = MeasureSpec.getMode(measureSpec);
	     int specSize = MeasureSpec.getSize(measureSpec);

	     if (specMode == MeasureSpec.EXACTLY) {
	         // We were told how big to be
	         result = specSize;
	     } else {
	         // Measure the text
	         result = mOval_r + getPaddingLeft()
	                 + getPaddingRight();
	         if (specMode == MeasureSpec.AT_MOST) {
	             // Respect AT_MOST value if that was what is called for by measureSpec
	             result = Math.min(result, specSize);
	         }
	     }
		 
	     return result;
	 }

	 /**
	  * Determines the height of this view
	  * @param measureSpec A measureSpec packed into an int
	  * @return The height of the view, honoring constraints from measureSpec
	  */
	 private int measureHeight(int measureSpec) {
	     int result = 0;
	     int specMode = MeasureSpec.getMode(measureSpec);
	     int specSize = MeasureSpec.getSize(measureSpec);

	     if (specMode == MeasureSpec.EXACTLY) {
	         // We were told how big to be
	         result = specSize;
	     } else {
	         // Measure the text (beware: ascent is a negative number)
	         result = mOval_b + getPaddingTop()
	                 + getPaddingBottom();
	         if (specMode == MeasureSpec.AT_MOST) {
	             // Respect AT_MOST value if that was what is called for by measureSpec
	             result = Math.min(result, specSize);
	         }
	     }
	     
	     return result;
	 }
}

在构造中getDimensionPixelOffset检索出一个属性值，没有的话就使用第2个参数做默认值，

可以在布局xml中初始化这个属性值，下面是我main.xml

[java] view plain copy print ?

1. <?xml version="1.0" encoding="utf-8"?>
2. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
3. xmlns:app="http://schemas.android.com/apk/res/com.myos"
4. android:orientation="vertical"
5. android:layout_width="fill_parent"
6. android:layout_height="fill_parent"
7. >
9. <TextView
10. android:layout_width="wrap_content"
11. android:layout_height="wrap_content"
12. android:text="@string/hello"
13. />
15. <com.myos.RocketView
16. app:ovalLeft="0dp"
17. android:id="@+id/rv"
18. android:layout_width="wrap_content"
19. android:layout_height="wrap_content" />
22. </LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res/com.myos"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
	
	<TextView  
	    android:layout_width="wrap_content" 
	    android:layout_height="wrap_content" 
	    android:text="@string/hello"
	    />
	
	<com.myos.RocketView
	    app:ovalLeft="0dp"
	    android:id="@+id/rv"
	    android:layout_width="wrap_content"
	    android:layout_height="wrap_content" />
	

</LinearLayout>

我这里没啥用处，仅仅试一下，待扩展属性时再说，说明一下xmlns:app中app可以随便命名的，这个android的规矩还真多

另外构造函数中onMeasure很重要的，onDraw很明显就是画椭圆了，android在画的时候呢，会去先测量，需要我们来提供View的宽和高，

大家都知道wrap_content、fill_parent，测量时会有3种模式，分别是UNSPECIFIED、EXACTLY、AT_MOST，

当使用

wrap_content时就是AT_MOST模式，fill_parent就是EXACTLY模式，可以看看官方文档：

http://developer.android.com/guide/topics/ui/how-android-draws.html

相信研读一下代码就明白了

RocketView定义好了，接下来就是使用了

[java] view plain copy print ?

1. package com.myos;
3. import android.app.Activity;
4. import android.os.Bundle;
6. public class MainActivity extends Activity {
7. RocketView mRocketView;
8. /** Called when the activity is first created. */
9. @Override
10. public void onCreate(Bundle savedInstanceState) {
11. super.onCreate(savedInstanceState);
12. setContentView(R.layout.main);
14. mRocketView = (RocketView)findViewById(R.id.rv);
15. mRocketView.setOvalRect(0, 0, 500, 100);
16. }
17. }

package com.myos;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {
	RocketView mRocketView;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        mRocketView = (RocketView)findViewById(R.id.rv);
        mRocketView.setOvalRect(0, 0, 500, 100);
    }
}

可以通过setOvalRect函数来动态调整椭圆的大小！

下面是运行效果，ko啦！

onMeasure方法在控件的父元素正要放置它的子控件时调用.它会问一个问题，“你想要用多大地方啊？”，然后传入两个参数——widthMeasureSpec和heightMeasureSpec.

　　它们指明控件可获得的空间以及关于这个空间描述的元数据.
　　比返回一个结果要好的方法是你传递View的高度和宽度到setMeasuredDimension方法里.
(请发邮件到 fafanqiang@126.com 获得翻强软件，能上youtube哟。)
　　接下来的代码片段给出了如何重写onMeasure.注意，调用的本地空方法是来计算高度和宽度的.它们会译解widthHeightSpec和heightMeasureSpec值，并计算出合适的高度和宽度值.

java代码：

1. @Override
   
2. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   
3. 
   
4. int measuredHeight = measureHeight(heightMeasureSpec);
   
5. int measuredWidth = measureWidth(widthMeasureSpec);
   
6. setMeasuredDimension(measuredHeight, measuredWidth);
   
7. }
   
8. 
   
9. private int measureHeight(int measureSpec) {
   
10. 
    
11. 
    
12. // Return measured widget height.
    
13. }
    
14. 
    
15. private int measureWidth(int measureSpec) {
    
16. 
    
17. // Return measured widget width.
    
18. }

复制代码

边界参数——widthMeasureSpec和heightMeasureSpec ，效率的原因以整数的方式传入。

MeasureSpec封装了父布局传递给子布局的布局要求，每个MeasureSpec代表了一组宽度和高度的要求。一个MeasureSpec由大小和模式组成。

它有三种模式：

UNSPECIFIED(未指定), 父元素不对自元素施加任何束缚，子元素可以得到任意想要的大小；

EXACTLY(完全)，父元素决定自元素的确切大小，子元素将被限定在给定的边界里而忽略它本身大小；

AT_MOST(至多)，子元素至多达到指定大小的值。

它常用的三个函数：

　　 1.static int getMode(int measureSpec):根据提供的测量值(格式)提取模式(上述三个模式之一)

　　2.static int getSize(int measureSpec):根据提供的测量值(格式)提取大小值(这个大小也就是我们通常所说的大小)

　　3.static int makeMeasureSpec(int size,int mode):根据提供的大小值和模式创建一个测量值(格式)

这个类的使用呢，通常在view组件的onMeasure方法里面调用但也有少数例外

在它们使用之前，首先要做的是使用MeasureSpec类的静态方法getMode和getSize来译解，如下面的片段所示：

java代码：

1. int specMode = MeasureSpec.getMode(measureSpec);
   
2. int specSize = MeasureSpec.getSize(measureSpec);

复制代码

依据specMode的值，如果是AT_MOST，specSize 代表的是最大可获得的空间；如果是EXACTLY，specSize 代表的是精确的尺寸；如果是UNSPECIFIED，对于控件尺寸来说，没有任何参考意义。
　　当以EXACT方式标记测量尺寸，父元素会坚持在一个指定的精确尺寸区域放置View。在父元素问子元素要多大空间时，AT_MOST指示者会说给我最大的范围。在很多情况下，你得到的值都是相同的。
　　在两种情况下，你必须绝对的处理这些限制。在一些情况下，它可能会返回超出这些限制的尺寸，在这种情况下，你可以让父元素选择如何对待超出的View，使用裁剪还是滚动等技术。

　　接下来的框架代码给出了处理View测量的典型实现：

java代码：

1. @Override
   
2. 
   
3. protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
   
4. 
   
5. int measuredHeight = measureHeight(heightMeasureSpec);
   
6. 
   
7. int measuredWidth = measureWidth(widthMeasureSpec);
   
8. 
   
9. setMeasuredDimension(measuredHeight, measuredWidth);
   
10. 
    
11. }
    
12. 
    
13. private int measureHeight(int measureSpec) {
    
14. 
    
15. int specMode = MeasureSpec.getMode(measureSpec);
    
16. int specSize = MeasureSpec.getSize(measureSpec);
    
17. 
    
18. // Default size if no limits are specified.
    
19. 
    
20. int result = 500;
    
21. if (specMode == MeasureSpec.AT_MOST){
    
22. 
    
23. // Calculate the ideal size of your
    
24. // control within this maximum size.
    
25. // If your control fills the available
    
26. // space return the outer bound.
    
27. 
    
28. result = specSize;
    
29. }
    
30. else if (specMode == MeasureSpec.EXACTLY){
    
31. 
    
32. // If your control can fit within these bounds return that value.
    
33. result = specSize;
    
34. }
    
35. 
    
36. return result;
    
37. }
    
38. 
    
39. private int measureWidth(int measureSpec) {
    
40. int specMode = MeasureSpec.getMode(measureSpec);
    
41. int specSize = MeasureSpec.getSize(measureSpec);
    
42. 
    
43. // Default size if no limits are specified.
    
44. int result = 500;
    
45. if (specMode == MeasureSpec.AT_MOST){
    
46. // Calculate the ideal size of your control
    
47. // within this maximum size.
    
48. // If your control fills the available space
    
49. // return the outer bound.
    
50. result = specSize;
    
51. }
    
52. 
    
53. else if (specMode == MeasureSpec.EXACTLY){
    
54. // If your control can fit within these bounds return that value.
    
55. 
    
56. result = specSize;
    
57. }
    
58. 
    
59. return result;
    
60. }